Method of biologically removing nitrogen and system therefor

ABSTRACT

In a method of biologically removing nitrogen and a system therefor according to the present invention, by utilizing the characteristics of AH bacteria as being nitrifying bacteria of nitrous acid type for displaying high activity at the high concentration of ammonium nitrogen and the characteristics of AL bacteria as being nitrifying bacteria of nitrous acid type for displaying high activity at the low concentration of the ammonium nitrogen, in the nitrifying process, operations capable of obtaining the maximum nitrifying speed in accordance with the concentration of the ammonium nitrogen can be performed, and the nitrifying reaction of nitrous acid type for producing nitrous acid as being an intermediate oxide material of the ammonium nitrogen can be performed.

This is a Division of application Ser. No. 08/682,019 filed Jul. 16,1996, pending. The entire disclosure of the prior application is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of biologically removing nitrogen anda system therefor, and more particularly to a system for biologicallyremoving nitrogen, in which waste water containing ammonium nitrogen athigh concentration is nitrified and denitrified by use of fixedmicroorganisms, whereby nitrogen contained in the waste water isremoved.

2. Description of the Related Art

Since ammonium nitrogen contained in the waste water becomes one of thematerials causing decrease of dissolved oxygen in a water area forpublic use, into which effluent flows in and also causing eutrophy in aclosed water area, it is necessary to remove ammonium nitrogen in thewaste water.

As a method of biologically removing ammonium nitrogen in the wastewater, normally, there is a process by use of a single sludgepredenitrification process, in which there are utilized nitrifyingreaction from ammonia to nitric acid by use of nitrifying bacteria anddenitrifying reaction from nitric acid to nitrogen by use ofdenitrifying bacteria. According to this method, two tanks including adenitrifying tank in the anaerobic condition and a nitrifying tank inthe aerobic condition are used, in the nitrifying tank, decomposition ofthe organic substance and the denitrifying process are performed by thedenitrifying bacteria, and, in the nitrifying tank, ammonium nitrogen inthe waste water is nitrified into nitric acid by the nitrifyingbacteria. Then, a nitrified liquid which is nitrified in the nitrifyingtank is recycled through the denitrifying tank, whereby a nitrogencomponent in the waste water is discharged into condition as nitrogengas and removed. In this single sludge predenitrification process, inorder to improve the nitrifying efficiency, fixing of the nitrifyingbacteria is examined, and a process, in which the fixed nitrifyingbacteria are thrown into the nitrifying tank is used in practice.Further, a process, in which the fixed denitrifying bacteria are throwninto the denitrifying tank, is examined.

The waste water, to which this single sludge predenitrification processis applied, is chiefly sewerage having the ammonium nitrogenconcentration (NH₄ --N) as low as about 20˜60 mg/l, and, in the ammoniumnitrogen concentration as low as this extent, nitrifying reactionproceeds smoothly, whereby the ammonium nitrogen concentration in theeffluent is decreased to several mg/l less than the effluent standard inthe river area.

However, in the case where the waste water containing the high ammoniumnitrogen concentration (approximately, 400 mg/l˜5000 mg/l) is producedin great quantity such for example as in a developing laboratory, aninorganic matter synthesizing factory, a power plant and the like, theammonium nitrogen concentration of the original waste water is dilutedto less than 200 mg/l, and then, the biological process is performed. Asthe result, such a problem occurs that the amount of the waste water tobe processed is increased considerably, whereby a large-sized system forbiologically removing nitrogen is needed, however, it is difficult toinstall the large-sized system in the cities where any largeinstallation area cannot be obtained.

As a method of biologically processing the ammonium nitrogen waste waterhaving the high concentration, the inventors of the invention of thepresent application have previously developed a method of processing athigh speed by a multiple stage process by use of microorganismsentrapped in organic gel. According to this method, the waste water iscaused to flow in series through three nitrifying tanks (aerationtanks), into which media of microorganisms entrapped in organic gel arethrown. For example, in the case where operating conditions are selectedsuch that the ammonium nitrogen concentration of the waste water (theammonium nitrogen concentration of the waste water, which was caused toflow into a first thank) was 250 mg/l, the amount of the processed wastewater was 480 m³ /day and the residence time was 7 hours, the quality ofthe processed water in a third tank was decreased to a value as low as37.5 mg/l. Furthermore, since the nitrifying speed of media in the firsttank has shown a very high value of 123 mg-N/h·l-medium, this fact washighly noted as one which made the system to be compact in size. Thisnitrifying speed is an important factor highly influencing on the rateof removal of nitrogen for determining a required capacity of anaeration thank. The higher the nitrifying speed is, the more the systemcan be made compact in size.

As described above, necessity has been voiced for making the system forbiologically removing nitrogen to be compact in size. In particular,when the system for biologically removing nitrogen for processing theammonium nitrogen waste water having the high concentration can be madeto be compact in size, the usefulness is very high.

However, in the above-described case of multiple stage process, when thewaste water, in which the ammonium nitrogen concentration is as high asseveral thousand mg/l, there is such a problem that the dilutionmagnification is still required to be increased, and it is stillunsatisfactory for making the system to be compact in size.

Because of this, necessity has been voiced for the development of asystem for biologically removing nitrogen capable of highly processing,in which the ammonium nitrogen waste water having the high concentrationcan be processed as it is not diluted or at a concentration of a lowdilution magnification.

Now, as for the nitrifying and denitrifying processes, such a concept isgeneralized that in the nitrifying process, the ammonium nitrogen isoxidized into nitrous acid by bacteria which are generally calledNitrosomonas, then, oxidized into nitric acid by Nitrobactor, andsubsequently, reduced into nitrogen gas in the denitrifying process.However, if, at the stage of nitrous acid which is an intermediate oxidematerial in the nitrifying process, nitrous acid is moved todenitrifying process, i.e., nitrifying and denitrifying reactions ofnitrous acid type can be carried out, then, the reaction process can beshortened and the processing time can be decreased according, so thatthis fact contributes to making the system compact in size, andmoreover, it is expected to raise the reacting speed.

SUMMARY OF THE INVENTION

The present invention has been developed to obviate the above-describeddisadvantages and has as its object the provision of realizing a methodof biologically removing nitrogen and a system therefor, in which thesystem for removing nitrogen from the waste water of ammonium nitrogenis made highly compact in size.

The inventors of the present invention have found that both AH bacteriaand AL bacteria are ones for oxidizing ammonium nitrogen into nitrousacid, under the conditions where these AH bacteria and AL bacteriapreferentially grow, the rate of nitrous acid is high in the wholenitric oxide materials (the total of nitrous acid and nitric acid)produced by oxidation of the ammonium nitrogen, and further, when liquidcontaining high nitrous acid is denitrified, nitrous acid-reducingbacteria, which are denitrifying bacteria of the type of denitrifyingnitrous acid, grow preferentially, and have developed the invention onthese knowledge.

As an example, in the liquid containing the ammonium nitrogen, which hasflowed into an effluent portion, such a nitrifying process is performedthat the rate of nitrous acid as being an intermediate oxide material ishigher than that of nitric acid as being the final oxide with saidliquid contacting under the aerobic condition in a reaction portioncontaining media, in which AH bacteria have grown preferentially.Further, the nitrified liquid, in which the rate of nitrous acid ishigher, is put under reaction in the reaction portion containing thedenitrifying bacteria, whereby the denitrifying bacteria of the type ofdenitrifying nitrous acid grow preferentially, so that nitrous acid canbe reduced into nitrogen gas and removed.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of this invention, as well as other objects and advantagesthereof, will be explained in the following with reference to theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures and wherein:

FIG. 1 is a drawing showing the relationship between the nitrifyingspeed and the number of AH bacteria against the ammonium nitrogenconcentration,

FIG. 2 is a drawing showing the relationship between the nitrifyingspeed and the number of AL bacteria against the ammonium nitrogenconcentration,

FIG. 3 is a drawing showing the relationship between the nitrifyingspeeds of AH bacteria and AL bacteria against the ammonium nitrogenconcentration,

FIG. 4 is a drawing showing the relationship between the number ofbacteria and the rate of nitrous acid out of the whole nitric oxidematerials as being products of reaction of nitrifying reaction when theammonium nitrogen load per medium is changed between 100˜600mg-N/h·l-medium while the ammonium nitrogen concentration is kept at 200mg/l,

FIG. 5 is a drawing showing the relationship between the number ofbacteria and the rate of nitrous acid out of the whole nitric oxidematerials when the ammonium nitrogen load per medium is changed between100˜600 mg-N/h·l-medium while the ammonium nitrogen concentration iskept at 500 mg/l,

FIG. 6 is a block diagram, in which nitrifying tanks are provided at twostages in series in the system of biologically removing nitrogenaccording to the present invention,

FIG. 7 is a block diagram, in which nitrifying tanks are provided atthree stages in series in the system for biologically removing nitrogenaccording to the present invention,

FIG. 8 is a block diagram, in which nitrifying tanks are provided atthree stages in series and the third stage is made to be an activatedsludge tank in the system for biologically removing nitrogen accordingto the present invention,

FIG. 9 is a block diagram, in which the system for biologically removingnitrogen according to the present invention consist of nitrifying anddenitrifying,

FIG. 10 is a block diagram, in which the system for biologicallyremoving nitrogen according to the present invention consists ofnitrifying, denitrifying and nitrifying,

FIG. 11 is a block diagram, in which the system for biologicallyremoving nitrogen according to the present invention consists ofnitrifying, denitrifying, nitrifying and denitrifying,

FIG. 12 is a block diagram, in which the system for biologicallyremoving nitrogen according to the present invention consists ofnitrifying, denitrifying, nitrifying, denitrifying and nitrifying,

FIG. 13 is a block diagram, in which the system for biologicallyremoving nitrogen according to the present invention consists ofnitrifying, denitrifying, nitrifying, denitrifying, nitrifying anddenitrifying,

FIG. 14 is a block diagram, in which the system for biologicallyremoving nitrogen consists of nitrifying, nitrifying and denitrifying,

FIG. 15 is a block diagram, in which reaction tanks consisting of pairsof AH bacteria nitrifying tanks and denitrifying tanks are provided at aplurality of stages in series,

FIG. 16 is a block diagram for explaining a first from of the system forbiologically removing nitrogen with a device for thickening the ammoniumnitrogen according to the present invention,

FIG. 17 is a block diagram for explaining a second from the system forbiologically removing nitrogen with a device for thickening the ammoniumnitrogen according to the present invention,

FIG. 18 is a view showing the operating conditions and the result in anembodiment, in which the first form of the system for biologicallyremoving nitrogen with the device for thickening the ammonium nitrogenaccording to the present invention is used, and

FIG. 19 is a view showing the operating conditions and the result in anembodiment, in which the first form of the system for biologicallyremoving nitrogen with the device for thickening the ammonium nitrogenaccording to the present invention is used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is developed on the basis of the knowledge of thatAH bacteria and AL bacteria, which will be described hereunder, areincluded in the types of the nitrifying bacteria. Firstly, AH bacteriaand AL bacteria, which are needed for understanding the presentinvention, will be described before describing a system for processingwaste water according to the present invention.

That is, when media carrying nitrifying bacteria were thrown into threetanks and three stage process, in which ammonium nitrogen waste waterwas caused to flow through three tanks in order to achieve an object ofspeedifying the nitrifying process to a high speed, the inventors of thepresent invention were able to obtain a nitrifying speed as high as 123mg-N/h·l-medium. Then, in order to find the cause of this fact, theinventors of the present invention have purified the nitrifying bacteriacontained in the media and examined the characteristics of the bacteria.As the result, the inventors of the present invention have broadlydivided the bacteria generally called nitrifying bacteria and found thatthere are living the nitrifying bacteria (AH bacteria) displaying highactivity in the high concentration ammonium nitrogen condition and thenitrifying bacteria (AL bacteria) displaying high activity in the lowconcentration ammonium nitrogen condition. The both AH bacteria and ALbacteria are the bacteria for oxidizing the ammonium nitrogen intonitrous acid as being an intermediate oxide material.

Then, the inventors of the present invention have tried to specify thesetwo types of the nitrifying bacteria, specified the nitrifying bacteriadetected after being cultured for eight weeks in ammonium sulfate liquidhaving the high concentration of 5000 mg/l as AH bacteria, and specifiedthe nitrifying bacteria detected after being cultured for eight weeks inammonium sulfate having the low concentration of 100 mg/l as ALbacteria.

In order to search the characteristics of AH bacteria and AL bacteria,which are thus specified, the nitrifying speed of the media and thedensity of the bacteria which are alive in the media when AH bacteriaand AL bacteria are cultured while the ammonium nitrogen concentrationis changed are measured in detail, and the relationships shown in FIGS.1 and 2 have been obtained.

FIG. 1 shows the number of bacteria and nitrifying speed of AH bacteriafor the ammonium nitrogen concentration, and FIG. 2 shows the number ofbacteria and the nitrifying speed of AL bacteria for the ammoniumnitrogen concentration. The nitrifying speed is an important factor fordetermining the required capacity of the nitrifying tanks and forinfluencing on the rate of removing nitrogen, and the higher thenitrifying speed is, the more the system can be made compact in size.

As known from FIG. 1, AH bacteria have the number of bacteria and thenitrifying speed are low when the ammonium nitrogen concentration is 200mg/l or less. However, when the ammonium nitrogen concentration is 400mg/l or more, the number of bacteria is increased to a number in doublefigures and the nitrifying speed per medium is increased 300mg-N/h·l-medium to a value more than three times of that at the time ofthe low concentration.

As known from FIG. 2, AL bacteria have the high number of bacteria whenthe ammonium nitrogen concentration is 200 mg/l or less, with the peakof the nitrifying speed being near 100 mg/l. Then, in the region of lessthan 100 mg/l, the rate of diffusion of ammonia controls nitrifing speedwithin the rate-determining step, with the apparent nitrifying speedbeing decreased. Furthermore, the nitrifying speed is decreased in theregion of 100 mg/l or more. It is presumable that this is caused by thebacteria are poisoned by ammonia.

FIG. 3 shows the nitrifying speed when nitrifying bacteria are culturedwithin the range of 150˜400 mg/l, with the mixing growth between AHbacteria and AL bacteria being recognized. During this time, thenitrifying speed reaches a value as high as 300˜450 mg-N/h·l-medium,with the combined effect due to the mixing of the two types of bacteriabeing recognized.

Furthermore, since the load conditions of the ammonium nitrogen permedium are important for the conditions of culturing and maintaining thebacteria, the characteristics of AH bacteria and AL bacteria aresearched under the coexistence with the nitrous acid oxidizing bacteriaas being the nitrifying bacteria for oxidizing nitrous acid into nitricacid as being the finial oxide while changing the load conditions of theammonia nitrogen under the conditions where the ammonium nitrogenconcentration is maintained at a predetermined level. FIG. 4 shows thenumber of bacteria and the rate of nitrous acid out of the whole nitricoxide materials (the total of nitrous acid and nitric acid) as being thereaction product of the nitrifying reaction when the load of ammoniumnitrogen per medium is changed between 100˜600 mg-N/h·l-medium while theammonium nitrogen concentration is maintained at 200 mg/l. FIG. 5 showsthe number of bacteria and the rate of nitrous acid out of the wholenitric oxide materials when the load of ammonium nitrogen per medium ischanged between 100˜600 mg-N/h·l-medium while the ammonium nitrogenconcentration is maintained at 500 mg/l.

As known from FIG. 4, when the culture was performed at the ammoniumnitrogen concentration of 200 mg/l, a mixed biota which were consistedof AH bacteria, AL bacteria and the nitrous acid oxidizing bacteria wasobserved. It has been found that, when the load of ammonium nitrogen permedium is 300 mg-N/h·l-medium or less, the number of bacteria is as lowas 10⁷, however, when the load of ammonium nitrogen per medium isincreased, the number of bacteria is increased remarkably, when the loadof ammonium nitrogen per medium is 400 mg-N/h·l-medium or more, thenumber of bacteria can be maintained at 10⁸ or more, and, when the loadof ammonium nitrogen per medium is 500 mg-N/h·l-medium or more, thenumber of bacteria can be maintained at 10⁹ or more. On the other hand,it has been found that, when the load of ammonium nitrogen per medium isincreased, the rate of nitrous acid out of the whole nitric oxidematerials is increased in parallel to the remarkable increase of thenumber of bacteria, and, when the load of ammonium nitrogen per mediumis made to be 500 mg-N/h·l-medium or more, the rate of nitrous acidbecomes about 100%.

Furthermore, as known from FIG. 5, when culture was performed at theammonium nitrogen concentration of 500 mg/l, a single biota containingonly AH bacteria was resulted. Then, the number of bacteria, the rate ofnitrous acid out of the whole nitric oxide materials and the load ofammonium nitrogen per medium are increased, the number of bacteria isremarkably increased and the rate of nitrous acid is also remarkablyincreased, so that the result similar to the case of FIG. 4 can beobtained.

Accordingly, it has been found from FIGS. 4 and 5 that, despite ofeither the mixed biota between AH bacteria, AL bacteria and the nitrousacid oxidizing bacteria or the mixed biota between AH bacteria and thenitrous acid oxidizing bacteria, AH bacteria and AL bacteria growpreferentially and the density of bacteria is increased by increasingthe load of ammonium nitrogen per medium, with the result that thenitrifying speed per medium is increased. Furthermore, by increasing theload of ammonium nitrogen per medium, AH bacteria and AL bacteria, whichare the nitrifying bacteria for oxidizing the ammonium nitrogen intonitrous acid, grow preferentially, whereby AH bacteria and AL bacteriagovern the nitrifying reaction, so that the nitrifying reaction ofnitrous acid type can take precedence.

In the case of immobilized pellets "activated sludge entrapped inorganic gel" by increasing the load of ammonium nitrogen per medium, theammonium nitrogen, which is a nourishment source, can be suppliedsatisfactorily to the inside of the fixed media, whereby this leads tothe increase of the number of bacteria, the increase of the nitrifyingspeed per medium and further, the preference of the nitrifying reactionof nitrous acid type.

From the above-described examination the following can be said about AHbacteria and AL bacteria.

(1) The bacteria generally called the nitrifying bacteria as beingammonia oxidizing bacteria can be broadly divided into AH bacteriadisplaying high activity under the conditions of ammonium nitrogenhaving the high concentration and AL bacteria displaying high activityunder the conditions of ammonium nitrogen having the low concentration.These bacteria are nitrifying bacteria for oxidizing ammonium nitrogeninto nitrous acid.

(2) AH bacteria preferentially grow in the region of ammonium nitrogenconcentration of 400 mg/l or more, the number of bacteria is increasedremarkably, the nitrifying speed is raised remarkably, and at the sametime, reaction of nitrous acid type is performed in the presence of thenitrous acid oxidizing bacteria. However, in the region where theammonium nitrogen concentration is low, the nitrifying capacity nearlydisappears. This seems to be the characteristics of AH bacteria whichhave the activity under the high concentration, but have no activityunder the low concentration.

(3) AL bacteria preferentially grow in the region of the ammoniumnitrogen concentration of 200 mg/l or less, the number of bacteria isincreased remarkably, and the nitrifying speed shows a parabola havingthe peak at about 100 mg/l.

(4) In the region where the ammonium nitrogen concentration is 150˜400mg/l, AH bacteria and AL bacteria grow mixedly, the nitrifying speed hasa combined effect due to the mixing of the two types of bacteria (Referto FIG. 3), and at the same time, the reaction of nitrous acid type isperformed in the presence of the nitrous acid oxidizing bacteria.

(5) Despite of either the mixed biota between AH bacteria, AL bacteriaand the nitrous acid oxidizing bacteria or the mixed biota between AHbacteria and the nitrous acid oxidizing bacteria, by increasing the loadof ammonium nitrogen per medium, AH bacteria and AL bacteriapreferentially grow and the numbers of bacteria are increased, with theresult that the nitrifying speed per medium is increased, and at thesame time, AH bacteria and AL bacteria, which are the bacteria foroxidizing the ammonium nitrogen into nitrous acid, grow preferentially,whereby AH bacteria and AL bacteria govern the nitrifying reaction, sothat the nitrifying reaction of nitrous acid type take precedence.

When the above-described know ledges (1) to (5) are combinedly examined,by bringing liquid containing ammonia, in which the ammonium nitrogenconcentration is 400 mg/l or more in the liquid, into contact with thefixed media, in which AH bacteria as being the nitrifying bacteria growpreferentially, in the aerobic condition, the ammoniac waste waterhaving the high concentration is processed in the state of being at highconcentration, and the nitrifying reaction of nitrous acid type, whichproduces nitrous acid as being an intermediate oxide of the ammoniumnitrogen, can be carried out. By carrying out the nitrifying reaction ofnitrous acid type, the loading of addition of organic substance requiredfor the denitrifying process can be decreased as compared with the casewhere the conventional nitrifying reaction of nitric acid type iscarried out. In this case, when it is tried to make the load of ammoniumnitrogen per medium of the a fixed media, in which the aforesaid AHbacteria grow preferentially, to be 500 mg-N/h·l-medium or more,preferably 600 mg-N/h·l-medium or more, the nitrifying speed is furtherraised, more speedified process can be performed, and at the same time,the nitrifying reaction of nitrous acid type can govern. When the air isstrongly aerated to the waste water, the ammonia in the ammoniac wastewater having the high concentration is released in the air. As a result,the ammonia decreases slightly. In this case, however, there is asecondary problem.

Accordingly, in order to biologically remove the ammonia in the ammoniacwaste water having the high concentration, the desirable decrease of theammoniac nitrogen is less than 800 mg/l per process.

Furthermore, by bringing liquid containing ammonia, in which theammonium nitrogen concentration is 100˜400 mg/l or more in the liquid,into contact with the fixed media, in which AH bacteria and AL bacteria,which are the nitrifying bacteria grow mixedly, in the aerobiccondition, the ammoniac waste water ranging from the mediumconcentration to the low concentration can be processed at high speed,and the nitrifying reaction of nitrous acid type for producing nitrousacid as being an intermediate oxide material of the ammonium nitrogencan be carried out. Then, by carrying out the nitrifying reaction ofnitrous acid type, the loading of addition of organic substance requiredfor the denitrifying process can be decreased as compared with the casewhere the conventional nitrifying reaction of nitric acid type. In thiscase too, when it is tried to make the ammonium nitrogen per medium ofthe fixed media, in which the aforesaid AH bacteria and AL bacteria aremixed and fixed, to be 500 mg-N/h·l-medium or more, preferably 600mg-N/h·l-medium or more, the nitrifying speed is further raised, morespeedified process can be performed, and at the same time, thenitrifying reaction of nitrous acid type can govern.

Accordingly, by skillfully utilizing the characteristics of AH bacteriasuitable for the waste water of ammonium nitrogen having the highconcentration and the characteristics of AH bacteria plus AL bacteria,in which AH bacteria and AL bacteria, which are suitable for the wastewater of ammonium nitrogen having the medium concentration, growmixedly, the waste waters including the waste water of ammonium nitrogenhaving the high concentration at a developing laboratory, an inorganicmatter synthesizing factory, a power plant and the like and the wastewater of ammonium nitrogen having the low concentration in a sewerageand the like can be processed efficiently and at high speed, and at thesame time, the nitrifying reaction of nitrous acid type can be carriedout, so that the system for biologically removing nitrogen can be madecompact in size greatly.

FIGS. 6 to 14 are examples of arrangement of the system for biologicallyremoving nitrogen according to the present invention, which areconstructed on the basis of the above-described knowledges.

FIGS. 6 to 8 show the cases where the nitrifying tanks are provided atmultiple stages.

FIG. 6 shows the two stage process in which the waste water of ammoniumnitrogen is caused to flow through two nitrifying tanks in series, andFIG. 7 is the example of three stage process, in which the waste waterof ammonium nitrogen is caused to flow through three nitrifying tanks inseries. Furthermore, FIG. 8 is the example, in which the nitrifying tankof activated sludge method and the settling tank is provided in place ofthe nitrifying tank at the third stage shown in FIG. 7. In theserespective nitrifying tanks, there are used fixed media, in which AHbacteria preferentially grow in accordance with the concentration of theammonium nitrogen which flows into the respective nitrifying tanks,fixed media, in which AH bacteria and AL bacteria grow mixedly and fixedmedia, in which AL bacteria grow preferentially.

With this arrangement, in the respective nitrifying tanks, there areperformed operations capable of obtaining the maximum nitrifying speedin accordance with the ammonium nitrogen concentration, so that thenitrifying reaction can be carried out efficiently, the ammoniumnitrogen concentration can be effectively decreased and the process athigh speed can be performed.

Furthermore, FIGS. 9 to 14 are the examples in the case of consisting ofthe nitrifying tank or tanks and the denitrifying tank or tanks. FIG. 9shows the case where the system consists of the nitrifying tank and thedenitrifying tank, and the waste water is caused to flow through thenitrifying tank and the denitrifying tank in series, in which, as forthe fixed media which are thrown into the nitrifying tank, in the casewhere the ammonium nitrogen concentration of the waste water is high,fixed media, in which AH bacteria grow preferentially are thrown, and inthe case where the ammonium nitrogen concentration is from medium tolow, the fixed media, into which AH bacteria and AL bacteria growmixedly, are thrown. Furthermore, the floating type activated sludgecontaining the denitrifying bacteria or the fixed media of thedenitrifying bacteria are thrown into the denitrifying tank, and acarbon source such as methanol as being a hydrogen donor source for thedenitrifying bacteria is added to a nitrified liquid which is caused toflow from the nitrifying tank to the denitrifying tank. The reason whythe carbon source is added to the denitrifying tank resides in that,when the waste water is caused to flow from the nitrifying tank to thedenitrifying tank, the organic substance contained in the waste water(which is the hydrogen donor source for the denitrifying bacteria) isalso oxidized and decreased, so that the hydrogen donor source isrequired to be added to the nitrified liquid. However, from theviewpoint of economics, it is desirable to decrease the loading ofaddition as much as possible. Instead of adding the carbon source suchas methanol as being the hydrogen donor source, a bypass line from theoriginal waste water to the denitrifying tank may be provided to supplypart of the organic substance contained in the waste water to thedenitrifying tank.

FIG. 10 shows the arrangement, in the case where the second nitrifyingtank is further provided at the latter stage of the denitrifying tankshown in the arrangement of FIG. 9 and the nitrified liquid from thesecond nitrifying tank is recycled through the denitrifying tank. Inthis arrangement, as the fixed media, which are thrown into thenitrifying tank, in the case where the ammonium nitrogen concentrationof the waste water is high, the fixed media, in which AH bacteria growpreferentially in the first nitrifying tank, are thrown into the firstnitrifying tank, and fixed media, in which AH bacteria and AL bacteriagrow mixedly, are thrown into the second nitrifying tank. Furthermore,when the ammonium nitrogen concentration is from medium to low, fixedmedia, in which AH bacteria and AL bacteria grow mixedly, are throwninto the first nitrifying tank, and fixed media, in which AL bacteriagrow preferentially, are thrown into the second nitrifying tank.

FIG. 11 shows the case where the second denitrifying tank is furtherprovided at the latter stage of the second nitrifying tank shown in thearrangement of FIG. 10, the nitrified liquid from the second nitrifyingtank is not recycled through the first denitrifying tank, and thehydrogen donor source for the denitrifying bacteria is added at theformer stage of the second denitrifying tank.

FIG. 12 is the arrangement in the case where the third nitrifying tankis further provided at the latter stage of the second denitrifying tankin the arrangement shown in FIG. 11, and the nitrified liquid from thethird nitrifying tank is recycled through the second denitrifying tank.In this arrangement, as fixed media, which are thrown into thenitrifying tank, in the case where the ammonium nitrogen concentrationof the waste water is high, fixed media, in which AH bacteria growpreferentially, are thrown into he first nitrifying tank, and fixedmedia in which AH bacteria and AL bacteria grow prefentially, are throwninto the second nitrifying tank, and fixed media, in which AL bacteriagrow preferentially, are thrown into the third nitrifying tank.

FIG. 13 shows the case where the third denitrifying tank is furtherprovided at the latter stage of the third nitrifying tank in thearrangement of FIG. 12, the nitrified liquid from the third nitrifyingtank is not recycled through the second denitrifying tank, and thehydrogen donor source for the denitrifying bacteria is added at theformer stage of the third denitrifying tank.

FIG. 14 is a modification of FIG. 11 and shows the case where twonitrifying tanks for AH bacteria and for AH bacteria plus AL bacteriaare provided in series and one denitrifying tank is provided at thelatter stage of the two nitrifying tanks.

In the respective nitrifying tanks of the above-descried arrangement, inaccordance with the concentration of the ammonium nitrogen which flowsinto the respective nitrifying tanks, the type of fixed media to be used(fixed media, in which AH bacteria grow preferentially, fixed media, inwhich AH bacteria and AL bacteria are mixed and fixed media, in which ALbacteria grow preferentially) can be determined.

In the above-described FIGS. 6 to 14, in the case where the ammoniumnitrogen concentration is 400 mg/l or more, description has been givenof the example where there is provided only the first nitrifying tank,into which fixed media, in which AH bacteria grow preferentially, arethrown.

However, when the liquid containing the very high ammonium nitrogenconcentration as in the first nitrifying tank is nitrified, nitrous acidand nitric acid are produced in large quantities, whereby the liquid inthe first nitrifying tank is lowered in PH. The optimal range of PH ofthe nitrifying reaction is 7˜8. When PH is lowered to 6, the speed ofreaction is decreased to the half, and, when PH is less than 5.5, thereaction is stopped. Because of this, an alkali agent such as causticsoda should be added to raise PH. However, if the loading of addition ofthe alkali agent is increased, there occurs a problem of economics.Then, as shown in FIG. 15, the first nitrifying tank, into which thefixed media of AH bacteria are thrown, is divided into multi-stages, andthe denitrifying tanks are clamped between the respective stages. Thatis, a denitrifying tank is provided at the latter stage of a nitrifyingtank to form a single reaction tank, and the respective reaction tanksare provided at a plurality of stages in series. With this arrangement,in the state of PH of the liquid is raised by hydroxide ions produced bydenitrifying reaction of nitrous acid ions and nitric acid ions,nitrifying reaction in the nitrifying tank at the following stage can becontinued, so that the loading of addition of alkali agent can bedecreased greatly. With this arrangement, economical operations can becarried out.

As described above, in the system for biologically removing nitrogenaccording to the present invention, by utilizing the characteristics ofAH bacteria as being the nitrifying bacteria of nitrous acid type, whichdisplays high activity at the high concentration and the characteristicsof AL bacteria as being the nitrifying bacteria, which display highactivity at the low concentration, operations capable of obtaining themaximum nitrifying speed in accordance with the ammonium nitrogenconcentration during the nitrifying process and the nitrifying reactionof nitrous acid type for producing nitrous acid as being an intermediateoxide material of the ammonium nitrogen can be carried out. On the otherhand, in the denitrifying process, the nitrified liquid high in the rateof nitrous acid is denitrified, whereby the nitrous acid reducingbacteria as being the denitrifying bacteria of the type of denitrifyingnitrous acid grow preferentially, so that the denitrifying reaction forreducing nitrous acid into nitrogen gas can be efficiently carried out.

Description will hereunder be given of the detailed embodiments of thepresent invention, which are carried out by use of the system forbiologically removing nitrogen having the arrangement of the nitrifyingtanks and the denitrifying tanks as described above.

(Embodiment 1)

The embodiment 1 is an example, in which the waste water of ammoniumnitrogen having the high concentration is processed in the order ofnitrifying to denitrifying by use of the system for biologicallyremoving nitrogen as shown in FIG. 9.

Standard activated sludge in a sewerage processing place is used as seedbacteria for being fixed. The fixing is carried out such that activatedsludge 2 wt %, polyethylene glycol prepolymer 15 wt %, N, N, N',N'-tetramethyl ethylene diamin 0.5 wt % and potassium persulfate 0.25 wt% are mixed in the water and set up. Gel thus obtained is cut intomediums each having a square of 3 mm for use.

Fixed media each having the score of 3 mm (hereinafter referred to as"PEG gel media") are thrown into a nitrifying tank of 4 l, and thefill-up rate of PEG gel media is set at 10%. Under these conditions, theinorganic waste water containing the ammonium nitrogen concentration of2000 mg/l is processed for the residence time of 20 hours in anitrifying tank with daily load 2.4 kg-N/ m³ ·day (Load conversion permedium is 1000 mg-N/h·l-medium). Furthermore, the residence time in adenitrifying tank of 4 little is set at 20 hours and the ratio (C/N)between the amounts of carbon as being the hydrogen donor source addedinto the denitrifying tanks and the amount of the waste water nitrogenis set at 0.5 and the results after two months of the continuousoperations are shown in Table 1. Incidentally, sponge of 40% is filledin the denitrifying tank as a filler material. However, any other fillermaterial, fixed media or floating type bacteria may be used. Methanolwas used as the added hydrogen donor source.

Similarly, No. 2 of Table 1 shows the case where the packing rate of thePEG gel media is set at 20%. No. 3 of Table 1 shows the results obtainedin the case where the fixed media of connecting type "PVA media" areused and operations are performed under processing conditions similar toNo. 1 and No. 2. The PVA media each have a spherical shape of 3 mmφ andhas the surface connected to activated sludge. No. 4 of Table 1 showsthe results obtained in the case where sponge media of the connectingtype are used and operations are performed under processing conditionssimilar to the previous case. The sponge media are of a cube of 5 mm andhave the surface connected to the activated sludge. The load of ammoniumnitrogen concentration per medium in these No. 1˜No. 4 are set at 600mg-N/h·l-medium or more.

A comparative example is obtained under the conditions, in whichoperations are performed by use of the activated sludge of floating typein both the nitrifying tank and denitrifying tank and the activatedsludge is held at 4000 mg/l.

Table 1 shows the results of No. 1˜No. 4 and the comparative example.

It is known from Table 1 that, in the case of the floating typeactivated sludge in the comparative example, the process was performedfor two months, but the ammonium nitrogen concentration of the processedwater was 1965 mg/l not able to process practically.

In contrast thereto, in the embodiments Nos. 1 to 4 according to thepresent invention, the nitrifying speed is 215 mg-N/h·l-medium or more,particularly, in the case of the PEG gel media (packing rate 10%), thenitrifying speed is 560 mg-N/h·l-medium or more, a very high nitrifyingspeed is obtained, and nitrogen in the form of nitric acid out of thewhole nitric oxide materials in the processed water is 10 mg/l or less,so that it is estimated that the nitrifying reaction of nitrous acidtype governs.

As described above, the reason why the high nitrifying speed is obtainedand the nitrifying reaction of nitrous acid type governs is deemed toreside in that the load of ammonium nitrogen per medium is set at 600mg-N/h·l-medium or more, AH bacteria of nitrous acid type, which displayhigh activity at the high ammonium nitrogen concentration grow to a highdensity. When the number of bacteria in the media of Nos. 1 to 4 ispractically measured, all of the media have high concentration where AHbacteria are 10⁹ ˜10¹⁰ cell/cm³ -medium. Furthermore, the reason why No.1˜No. 4 differ in nitrifying speed is deemed to reside in thedifferences in the types of media and the packing rates.

                  TABLE 1                                                         ______________________________________                                                                    Nitrifying                                                        Effluent water                                                                            speed mg-                                                         (mg/l)      N/h · l -                                No.  Fixed Media      NH.sub.4 --N                                                                           NO.sub.3 --N                                                                         medium                                  ______________________________________                                        1    PEG gel media (Activated                                                                       880      <10    560                                          sludge entrapped in PEG gel)                                                  (10%)                                                                    2    PEG gel media (Activated                                                                       410      <10    398                                          sludge entrapped in PEG gel)                                                  (20%)                                                                    3    Connecting type PVA media                                                                      825      <10    294                                          (20%)                                                                    4    Connecting type sponge                                                                         1140     <10    215                                          media (20%)                                                              5    Activated sludge 1965     --     --                                           (MLSS4000 mg/l)                                                          ______________________________________                                    

In parentheses, packing rate of media and concentration of activatedsludge are shown.

(Embodiment 2)

The embodiment 2 is an example, in which the waste water of ammoniumnitrogen having the high concentration is processed through multiplestages in the order of nitrifying→denitrifying→nitrifying→denitrifying→nitrifying→denitrifying by use ofthe system for biologically removing nitrogen as shown in FIG. 13.

In the embodiment 2 also the activated sludge 2 wt % was used forforming fixed media similarly to the embodiment 1 and the fixed mediawere thrown into a first nitrifying tank, a second nitrifying tank and athird nitrifying tank. Furthermore, the denitrifying bacteria in thefirst, second and third denitrifying tanks are similar to those in theembodiment 1. Operating conditions were selected such that the ammoniumnitrogen concentration of the waste water was 500 mg/l, in both thenitrifying tanks and the denitrifying tanks, the first tank contained 4l, the second tank contained 4 l and the third tank contained 4 l, thetotal of the nitrifying tanks being 12 l, and the residence times in theportion of the nitrifying tanks were determined such that the first tankwas 4 hours, the second tank was 4 hours and the third tank was 4 hours,the total of the residence times being 12 hours. The ratio (C/N) betweenthe total of the amounts of carbon as being the hydrogen donor sourceadded in the denitrifying tanks and the amount of nitrogen of the wastewater was set at 0.9 and the continuous operations were performed fortwo months.

A comparative example was performed by the single stage process (onestage process was performed in the nitrifying tanks with the residencetimes being 12 hours).

The conditions and results in the respective nitrifying tanks in themulti-stage process are shown in Table

                  TABLE 2                                                         ______________________________________                                                        First  Second   Third                                                         nitrifying                                                                           nitrifying                                                                             nitrifying                                                    tank   tank     tank                                          ______________________________________                                        Residence time (h)                                                                              4        4        4                                         Packing rate of media (%)                                                                       20       20       20                                        Load (Kg-N/m.sup.3 · d)                                                                3.00     1.20     0.43                                      Load of media (mg-N/h · l-medium)                                                      625      250      90                                        Nitrifying speed (mgN/h · l-medium)                                                    37.5     160      88                                        Effluent water NH.sub.4 --N (mg/l)                                                              200      72       2                                         Effluent water NO.sub.3 --N (mg/l)                                                              <10      20       25                                        Effluent water NO.sub.2 --N (mg/l)                                                              280      95       50                                        ______________________________________                                    

From the results shown in Table 2, it was substantiated that, in thecase of the embodiment 2, similarly to the embodiment 1, the load of theammonium nitrogen per medium in the nitrifying tanks was set at 600mg-N/h·l-medium or more, whereby the high nitrifying speed as high as375 mg-N/h·l-medium was obtained. Furthermore, it was proved that, outof the whole nitric oxide materials, NO₃ --N was less than 10 mg/l andNO₂ --N was 280 mg/l, so that the nitrifying reaction of nitrous acidcan govern. Although the results in the second nitrifying tank and thethird nitrifying tank were not so much as in the first nitrifying tank,the nitrifying reaction governed. Furthermore, although it was notshown, it was found that in the finally processed water from the thirddenitrifying tank, NH₄ --N was 3 mg/l or less, NO₃ --N was 10 mg/l orless and NO₂ --N was 3 mg/l or less, so that, in the denitrifying tanks,nitrous acid was efficiently reduced into nitrogen gas. From this, itwas estimated that, by the nitrified liquid, in which nitrous acid had ahigh ratio, was denitrified in the denitrifying tank, so that thenitrous acid reducing bacteria as being the denitrifying bacteria forreducing nitrous acid grew preferentially.

On the other hand, in the comparative example, NH₄ --N in the processedwater was 240˜280 mg/l and the process was not able to be performed.

Furthermore, there was examined an example, in which the multi-stageprocess was performed by the use of the system for biologically removingnitrogen as shown in FIG. 14 as a modification of the multi-stageprocess in the order of nitrifying→nitrifying→denitrifying. Althoughthis example is different from the previous example in that two stageprocess of the nitrifying tanks plus one stage process of thedenitrifying tank constitute the operating conditions and alkali isadded to the respective nitrifying tanks, the other operating conditionsare similar. The reason why alkali is added to the nitrifying tanksresides in that the liquid in the nitrifying tanks is inclined to acidicside due to nitric acid and nitrous acid produced by the nitrifyingreaction, which is not desirable for the reaction of microorganisms. Asthe results, NH₄ --N became 10 mg/l and less. However, there has beenseen the tendency of increase of NO₃ --N contained in the nitrifiedliquid from the nitrifying tanks. For this, in order to make NO₃ --N inthe finally processed water to be 10 mg/l or less, the ratio (C/N)between the total of the amounts of carbon as being the hydrogen donorsource in the denitrifying tanks and the amount of nitrogen in the wastewater was required to be a value as high as 2.0.

(Embodiment 3)

The embodiment 3 is an example, in which the waste water of ammoniumnitrogen having the high concentration was processed in the multi-stageprocess by use of the system for biologically removing nitrogen as shownin FIG. 11 in the order ofnitrifying→denitrifying→nitrifying→denitrifying. In the embodiment 3also, the activated sludge 2 wt % was used for forming fixed mediasimilarly to the embodiment 1 and the fixed media were thrown into thefirst nitrifying tank and the second nitrifying tank. Furthermore, thedenitrifying bacteria in the first denitrifying tank and the seconddenitrifying tank were similar to those in the embodiment 1. Operatingconditions were selected such that the ammonium nitrogen concentrationof the waste water was 500 m/l in both the nitrifying tanks and thedenitrifying tanks, the first tank contained 4 l and the second tankcontained 9.5 l, the total being 13.5 l, and the residence times weredetermined such that the first tank was 4 hours and the second tank was9.5 hours, the total of the residence times being 13.5 hours. The ratio(C/N) between the total of the amounts of carbon as being the hydrogendonor source added in the denitrifying tanks and the amount of nitrogenof the waste water was set at 1.25 and the continuous operations wereperformed for two months.

The conditions and the results of experiments in the respectivenitrifying tanks in the multi-stage process in the embodiment 3 areshown in Table

                  TABLE 3                                                         ______________________________________                                                        First  Second   Finally                                                       nitrifying                                                                           nitrifying                                                                             effluent                                                      tank   tank     tank                                          ______________________________________                                        Residence time (h)                                                                              4        9.5      --                                        Packing rate of media (%)                                                                       20       20       --                                        Load (Kg-N/m.sup.3 · d)                                                                3.00     0.51     --                                        Load of media (mg-N/h · l-medium)                                                      625      105      --                                        Nitrifying speed (mg-N/h · l-medium)                                                   375      101      --                                        Effluent water NH.sub.4 --N (mg/l)                                                              200      8        <10                                       Effluent water NO.sub.3 --N (mg/l)                                                              <10      140      <5                                        Effluent water NO.sub.2 --N (mg/l)                                                              280      40       <3                                        ______________________________________                                    

As the result, in the case of the multi-stage process in the embodiment3, the load of the ammonium nitrogen per medium in the first nitrifyingtank was set at 600 mg-N/h·l-medium or more, the processing at highspeed was possible and the nitrous acid type governed in the nitrifyingreaction in the first nitrifying tank. However, the rate of the nitrousacid in the second nitrifying tank was lowered and the rate of nitricacid was highered, so that the loading of addition of the amounts of thehydrogen donor source added to the denitrifying tanks were required tobe high.

(Embodiment 4)

The embodiment 4 is an example, in which the waste water of ammoniumnitrogen having the medium concentration is processed through multiplestages in the order of nitrifying →denitrifying→nitrifying→denitrifyingby use of the system for biologically removing nitrogen as shown in FIG.11. In the embodiment 4 also, the activated sludge 2 wt % was used forforming fixed media similarly to the embodiment 1 and the fixed mediawere thrown into the first nitrifying tank and the second nitrifyingtank. Furthermore, the denitrifying bacteria in the first denitrifyingtank and the second denitrifying tank were similar to those in theembodiment 1. Operating conditions were selected such that the ammoniumnitrogen concentration of the waste water was 200 mg/l, in thenitrifying tanks, the first tank contained 4 l and the second tankcontained 7 l, the total being 11 l, and the residence times weredetermined such that the first tank was 3 hours and the second tank was7 hours, the total of the residence times being 10 hours. The ratio(C/N) between the total of the amounts of carbon as being the hydrogendonor source added in the denitrifying tanks and the amount of nitrogenof the waste water was set at 1.0 and the continuous operations wereperformed for two months. Table 4 shows the results of the experiments.

                  TABLE 4                                                         ______________________________________                                                        First  Second   Finally                                                       nitrifying                                                                           nitrifying                                                                             effluent                                                      tank   tank     tank                                          ______________________________________                                        Residence time (h)                                                                              3        7        --                                        Packing rate of media (%)                                                                       10       20       --                                        Load (Kg-N/m.sup.3 · d)                                                                1.60     0.43     --                                        Laad of media (mg-N/h · l-medium)                                                      670      89       --                                        Nitrifying speed (mg-N/h · l-medium)                                                   250      88       --                                        Effluent water NH.sub.4 --N (mg/l)                                                              125      2        <5                                        Effluent water NO.sub.3 --N (mg/l)                                                              20       110      <5                                        Effluent water NO.sub.2 --N (mg/l)                                                              50       15       <3                                        AH bacteria (cell/cm.sup.3 -medium)                                                             4.3 × 10.sup.9                                                                   8.5 × 10.sup.8                                                                   --                                        AL bacteria (cell/cm.sup.3 -medium)                                                             4.2 × 10.sup.9                                                                   3.6 × 10.sup.9                                                                   --                                        ______________________________________                                    

From the results shown in Table 4, it was found that, in the embodiment4 also, the load of the ammonium nitrogen per medium in the firstnitrifying tank was set at 600 mg-N/h·l-medium, so that a highnitrifying speed was obtained, high speed process was possible and, asfor the nitrifying reaction in the first nitrifying tank, the nitrousacid type governed. However, the rate of the nitrous acid in thenitrifying tank was decreased and the loading of addition of thehydrogen donor source was required to be increased. Furthermore, NO₃ --Nin the effluent water in the nitrifying tank reached 125 mg/l, AHbacteria and AL bacteria grew mixedly in the media, and the numbers ofAH bacteria and AL bacteria were in the order of 10⁹ and substantiallyequal to each other. In the process of the waste water of ammoniumnitrogen having the medium concentration, it is desirable to use fixedmedia, in which AH bacteria and AL bacteria are maintained at thenumbers equal to each other.

(Embodiment 5)

The embodiment 5 is an example of a so-called method of fixing thenitrifying bacteria by separated culture, in which AH bacteria areseparated from the activated sludge and fixed. AH bacteria purified fromthe standard activated sludge in a sewerage processing place as the seedbacteria for fixing. Fixing was performed such that AH bacteria 2 wt %,polyethylene glycol prepolymer 15 wt %, N, N, N', N' tetramethylethylene diamine 0.5 wt % and potassium persulfate 0.25% were mixed inthe water and set up. Gel thus obtained is cut into mediums each havinga square of 3 mm for use.

Fixed media of AH bacteria, each having a score of 3 mm (hereinafterreferred to as "PEG-AH media") are thrown into a nitrifying tank of 4 l,and the fill-up rate of the PEG-AH media is set at 20%. At the latterstage of this nitrifying tank, a nitrifying tank and a settling tankaccording to the method of standard activated sludge were provided.Then, the inorganic waste water having a nitrogen concentration of 500mg/l was processed for the residence time of 8 hours (4 hours in thenitrifying tank of the fixed media plus 4 hours in the nitrifying tankaccording to the method of standard activated sludge).

Furthermore, as a comparative example, a process only by the method ofstandard activated sludge (residence time is 8 hours) was performed.Table 5 shows the results.

                  TABLE 5                                                         ______________________________________                                                  Residence time                                                                         Load       Effluent water                                            (h)      (Kg-N/m.sup.3 · d)                                                              NH.sub.4 --N (mg/l)                             ______________________________________                                        This embodiment                                                                           8          1.5        less than 3                                 Comparative example                                                                       8          1.5        240˜410                               ______________________________________                                    

As apparent from Table 5, as the ammonium nitrogen concentration of theprocessed water in this embodiment, a value as low as 3 mg/l or less wasobtained. In contrast thereto, the ammonium nitrogen concentration ofthe processed water in the comparative example was 280˜410 mg/l, wherebythe processing capacity was low. From this, it is found that the mediawhich have fixed AH bacteria highly contribute to the nitrification ofthe ammoniac waste water having the high concentration.

Table 6 is one, in which the number of AH bacteria at the time of fixingis compared with the number of AH bacteria after the operations of onemonth in the above-described experiments.

                  TABLE 6                                                         ______________________________________                                        Number of AH bacteria at the time                                                               Number of AH bcteria after                                  of fixing (cell/cm.sup.3 -medium)                                                               one month (cell/cm.sup.3 -medium)                           ______________________________________                                        2.0 × 10.sup.3                                                                            6.3 × 10.sup.9                                        4.1 × 10.sup.4                                                                            2.8 × 10.sup.10                                       1.5 × 10.sup.5                                                                            1.5 × 10.sup.10                                       4.8 × 10.sup.6                                                                            4.3 × 10.sup.10                                       ______________________________________                                    

As apparent from Table 6, it is desirable to make the number of AHbacteria at the time of fixing to be 10⁴ cell/cm³ -medium or more,whereby a high nitrating speed can be expected.

(Embodiment 6)

In the embodiment 5, AH bacteria obtained by purifying the activatedsludge have been used. However, the embodiment 6 is an example, in whichAH bacteria are enrichingly cultured from the activated sludge, which isa so-called method of fixing the nitrifying bacteria by the enrichmentculture. The standard activated sludge collected in the sewerageprocessing place was batchingly cultured in a culturing liquid, in whichthe ammoniac concentration was 100, 200, 400, 500 and 1000 mg/l and theload was 0.5 kg-N/m³ ·d, and thus obtained sludge was entrapped andfixed by the method explained in the embodiment 4. After the sludge wasentrapped in gel and fixed, the sludge was batchingly cultured for 20days and the ratio between the numbers of AH bacteria and AL bacteriawas measured. Table 7 shows the results.

                  TABLE 7                                                         ______________________________________                                        Concentration of NH.sub.4 --N of                                              waste water used in enrichment                                                                   Ratio of numbers of bacteria                               culture (mg/l)     (AL bacteria/AH bacteria)                                  ______________________________________                                        (1)  100               100                                                    (2)  200               1                                                      (3)  400               0.8                                                    (4)  500               0.2                                                    (5)  1000              0                                                      ______________________________________                                    

As apparent from Table 7, for the enrichment culture of AH bacteria, itis desirable to set the ammonium nitrogen concentration at 400 mg/l ormore, preferably 500 mg/l or more. By the way, when the ammoniumnitrogen concentration was 1000 mg/l, AH bacteria were enrichmentcultured at 100%.

(Embodiment 7)

In the embodiment 7, experiments of processing were performed such thatthere were selected three types of entrapped and fixed media includingentrapped and fixed media having only AL bacteria cultured by (1) inTable 7 of the embodiment 6, fixed media, in which AL bacteria and AHbacteria are cultured by (3), coexist, and entrapped and fixed mediahaving only AH bacteria cultured by (5), and a nitrifying systemconsisting of three stages of nitrifying tanks. The fixed media havingonly AH bacteria were thrown into the first nitrifying tank, theentrapped and fixed media, in which AL bacteria and AH bacteria coexist,were thrown into the second denitrifying tank, and the entrapped andfixed media having only AL bacteria were thrown into the thirdnitrifying tank. Operating conditions were selected such that theammonium nitrogen concentration of the waste water was set at 1000 mg/l,in the nitrifying tanks, the first tank contained 4 l, the second tankcontained 4 l and the third tank contained 4 l, the total being 12 l,and the residence times were determined such that the first tank was 5.3hours, the second tank was 5.3 hours and the third tank was 5.3 hours,the total being 15.9 hours. Table 8 shows the results.

                  TABLE 8                                                         ______________________________________                                                        First  Second   Third                                                         nitrifying                                                                           nitrifying                                                                             nitrifying                                                    tank   tank     tank                                          ______________________________________                                        Residence time (h)                                                                              5.3      5.3      5.3                                       Packing rate of media (%)                                                                       20       20       20                                        Load (Kg-N/m.sup.3 · d)                                                                4.53     2.26     0.68                                      Load of media (mg-N/h · l-medium)                                                      943      472      142                                       Nitrifying speed (mg-N/h · l-medium)                                                   470      330      140                                       Effluent water NH.sub.4 --N (mg/l)                                                              500      150      less than 1                               ______________________________________                                    

As apparent from Table 8, by performing three stage process (the totalresidence time of three tanks is 15.9 hours), it was able to carry outhigh speed process. That is, the ammonium nitrogen concentration of theprocessed water in the first tank was 500 mg/l, and the nitrifying speedat this time was a high value of 470 mg-N/h·l-medium. Then, the ammoniumnitrogen of the processed water in the third tank was decreased to asatisfactorily low level of 1 mg/l or less.

Incidentally, in the embodiment 7, description has been given of theexample, in which the fixed media having only AH bacteria, the fixedmedia, in which AH bacteria and AL bacteria grew mixedly, and the fixedmedia having only AL bacteria were processed in the three stages. Thesefixed media can be combined in any form, and the number of processingstages should not necessarily be limited to three stages. Furthermore,in the embodiment 6, the enrichment purified AL bacteria and AH bacteriahave been used, but, purely separated AL bacteria and AH bacteria may beused. Furthermore, as gel used for fixing, there is no particular limit,and there may be used various polymeric materials such for example aspolyacril amido, polyethylene glycol, polyvinyl alcohol, agar-agar,carrageenan and alginic acid. Furthermore, in this embodiment,description has been given of a lateral type multi-stage process, inwhich the tanks are arranged in the lateral direction. However, avertical type multi-stage process, in which the tanks are arranged inthe vertical direction, may be adopted. Furthermore, AH bacteria and ALbacteria used for the method of biologically removing nitrogen accordingto the present invention may be made to grow in a practical equipment,or AH bacteria and AL bacteria which are cultured at the outside of thepractical equipment may be added to the practical equipment.

Hereinabove, description has been given of the example, in which theammonium nitrogen of the waste water having the high concentration wasnitrified and denitrified as at the high concentration. However,description will hereunder be given of the case where the waste water ofammonium nitrogen in a sewerage, which has the low concentration isthickened, and thereafter, nitrified and denitrified.

FIG. 16 shows a first form of the system for biologically removingnitrogen, in which the waste water of ammonium nitrogen having the lowconcentration is thickened, and thereafter, nitrified and denitrified.

As shown in FIG. 16, afar the solid substance in the waste water ispreviously separated by the system for separating solid from liquid, notshown, the waste water of the ammonium nitrogen is caused to flow into afirst device 14 for thickening ammonia through a waste water flow-inpipe 12 by use of a pressurizing pump, not shown. The first device 14for thickening is formed of a device of reverse osmosis film (RO film),in which the thickened water, in which the ammonium nitrogen has apredetermined concentration, is separated through a film from permeatedwater containing almost no ammonium nitrogen. The permeated water isdischarged as it is from a permeated water piping 16, and the thickenedwater is caused to flow into a nitrifying device 20 through a thickenedwater supply piping 18. Furthermore, a bypass piping 22 extending fromthe original water flow-in pipe 12 to the nitrifying device 20 withoutpassing through the device 14 for thickening is provided, and an on-offvalve 24 is provided in the bypass piping 22. With this arrangement,when there is no need for thickening the ammonium nitrogen concentrationof the waste water to a satisfactorily high level, the waste water iscaused to be flow into the nitrifying device 20 directly through thebypass piping 22.

The nitrating device 20 includes a nitrifying tank 26, fixed media 28for fixing the nitrifying bacteria which are thrown into the nitrifyingtank 26, an aeration piping 30 for supplying air into the nitrifyingtank 26 to form the aerobic condition, and a screen 32 for preventingthe fixed media 28 from flowing out of the nitrifying tank 26. As thefixed media 28 which are thrown into the nitrifying tank 26, to fixedmedia, in which AH bacteria grow preferentially or the fixed media, inwhich AH bacteria and AL bacteria grow mixedly, is used, whereby thenitrifying process of the thickened water is performed in the aerobiccondition. The nitrified liquid which as been nitrified is caused toflow into a denitrifying device 36 through a nitrified liquid piping 34.Incidentally, PH, in which nitrifying bacteria work effectively, isdesirable to be 7˜8, and alkali may be added to the nitrifying device 20by way of precaution against decrease of PH due to the nitric acidcaused by the nitrifying process.

The denitrifying device 36 includes a denitrifying tank 38, a wateragitating machine 40 and a hydrogen donor supply piping 42. In thedenitrifying tank 38, the activated sludge containing the denitrifyingbacteria float, the hydrogen donor source for the denitrifying bacteriasuch as alcohol is supplied by the hydrogen donor supply piping 42, andthe denitrifying process for the nitrified liquid is performed. Thusdenitrified liquid is caused to flow into a solid-liquid separating tank46 through a denitrified liquid piping 44, and supernatant liquid whichis solid-liquid separated is caused to flow into a second thickeningdevice 50 through a supernatant liquid piping 48.

The second device 50 for thickening includes a precision filtering film(MF film) device and a reverse osmosis film (RO film) device consistingof a plurality of stages and having a three construction. Thedenitrifyed liquid caused to flow under pressure to the second device 50for thickening is separated through a film into thickened water, inwhich the ammonium nitrogen remaining in the denitrified liquid isthickened, and permeated water not containing the ammonium nitrogen, andthe permeated water is discharged from a processed water piping 52. Onthe other hand, the thickened water reaches the supernatant liquidpiping 48 through a return piping 54 and a recycling piping 56 which isbranched from the intermediate portion of the return piping 54, isrecycled to the second device 50 for thickening to be thickened toattain a predetermined thickening magnification, and part of therecycled thickened water is returned to the nitrifying device 20 throughthe return piping 54. Flow control between the recycling quantity of thethickened water and the returning quantity returned to the nitrifyingdevice 20 is performed by a flow rate regulating valve 58 provided inthe recycling piping 56.

Furthermore, in order that the liquid in the denitrifying device 36 canbe recycled to the nitrifying device 20 as necessary, a piping 60 forthe recycling liquid is connected to the intermediate portion of thereturn piping 54 and a recycling pump 62 is provided in the piping 60for the recycling liquid.

Description will hereunder be given of the action of a device 10 forprocessing the waste water constructed as described above according tothe present invention by way of an example, in which the ammoniumnitrogen waste water having the low concentration (about 80 mg/l) in thesewerage is processed.

The ammonium nitrogen waste water having the low concentration isfirstly caused to flow into the first device 14 for thickening andseparated through a film into thickened water, in which the ammoniumnitrogen is thickened to a predetermined concentration, and permeatedwater almost not containing the ammonium nitrogen. The permeated wateris discharged as it is and only the thickened water is caused to flowinto the nitrifying device 20. With this arrangement, the thickenedwater reduced in amount in accordance with the thickening magnificationis caused to flow from the first device 14 for thickening to thenitrifying device 20.

In the nitrifying device 20, the thickened water thickened in the firstdevice 14 for thickening is brought into contact with the fixed media28, in which the nitrifying bacteria are fixed, in the aerobiccondition, whereby the nitrifying process is performed. As for the fixedmedia 28 for fixing the nitrifying bacteria, which are used for thisnitrifying process, when the ammonium nitrogen concentration of thethickened water is 400 mg/l or more, the fixed media 28, in which AHbacteria grow preferentially, may be used, and, when the concentrationat a level of about 150˜400 mg/l, the fixed media 28, in which AHbacteria and AL bacteria grow mixedly, may be used. As described above,the ammonium nitrogen waste water is raised in concentration to thelevel of concentration suitable for the fixed media 28, in which AHbacteria grow preferentially or the fixed media 28, in which AH bacteriaand AL bacteria grow mixedly, and the nitrifying process is performed,so that the number of bacteria in the fixed media 28 can be increasedremarkably and a high nitrifying speed can be obtained. In this case,the higher the ammonium nitrogen concentration per medium is, the highernitrifying speed can be obtained, so that it is desirable to raise theammonium nitrogen load per medium within the limit where the ammoniumnitrogen concentration of the processed water is not increased notably.To be concrete, a fill and residence time and the like of the fixedmedia to be thrown into the nitrifying tank 26 may be determined suchthat the ammonium nitrogen load becomes 500 mg-N/h·l-medium or more,preferably 600 mg-N/h·l-medium or more.

Subsequently, the nitrified liquid nitrated in the nitrifying device 20is caused to flow into the denitrifying device 36, where thedenitrifying process is carried out by the activated sludge containingthe denitrifying bacteria, and discharged into condition as the nitrogengas. The denitrified liquid denitrified in the denitrifying device 36 issolid-liquid separated in the solid-liquid separating tank 46, thesupernatant liquid of the denitrified liquid is caused to flow into thesecond device 50 for thickening, and the ammonium nitrogen remaining inthe denitrified liquid is removed. On the other hand, one part ofactivated sludge, which is settled at the bottom of the solid-liquidseparation tank 46 is returned to the nitrifying device 20 and the otherpart of the activated sludge is extracted to the outside of the systemas the surplus sludge.

Subsequently, the denitrified liquid caused to flow into the seconddevice 50 for thickening is recycled and thickened such that apredetermined thickening magnification can be attained, so that theammonium nitrogen can be separated through a film into thickened water,in which the ammonium nitrogen is thickened, and permeated water almostnot containing the ammonium nitrogen. The thickened water is returned tothe nitrifying device 20 and nitrified again. On the other hand, thepermeated water is discharged as the processed water. With thisarrangement, the ammonium nitrogen remaining in the denitrified liquidis removed nearly perfectly, so that the ammonium nitrogen concentrationin the processed water can be decreased as much as possible.

As described above, by utilizing the characteristics of the fixed media28, in which AH bacteria as being the nitrifying bacteria growpreferentially, or the fixed media 28, in which AH bacteria and ALbacteria grow mixedly, the waste water of the ammonium nitrogen havingthe low concentration can be thickened and nitrified, so that thequantity of water of the waste water processed in the nitrifying device20 can be decreased greatly. Further, the high speed processing at thehigh nitrifying speed can be performed as compared with the conventionalmethod of activated sludge, so that the device 10 for processing thewaste water can be made compact in size greatly.

FIG. 17 shows an example of a second form of the system for biologicallyremoving nitrogen, in which the waste water of the ammonium nitrogenhaving the low concentration is thickened, and thereafter, nitrified anddenitrified. The first device 14 for thickening and the second device 50for thickening are changed from the film separation type in the firstform to a distillation separation type. FIG. 17 shows only a nitrifyingsystem.

As shown in FIG. 17, in the waste water of the ammonium nitrogen, thesolid substance in the waste water is previously separated by asolid-liquid separating device, not shown, and thereafter, the wastewater is caused to flow into the first device 14 for thickening ammoniathrough the original water flow-in pipe 12. The first device 14 forthickening ammonia includes a distillation device 70, a condensationapparatus 72 and an absorbing tank 74, in which an absorbing liquid (forexample, a weak acid such as acetic acid) is stored. In the middle stageposition in the distillation device 70, a fill-up layer 76 filled withRasching ring is formed, the waste water of the ammonium nitrogen isshowered through a showering piping 78 provided upwardly of the fill-uplayer 76, and heated air is supplied to the fill-up layer 76 through aheated air piping 80 provided downwardly of the fill-up layer 76. Withthis arrangement, the waste water of the ammonium nitrogen is separatedthrough distillation into distilled water, in which the ammoniumnitrogen is thickened, and residue of distillation almost not containingthe ammonium nitrogen. The distilled liquid is distilled in thecondensation thickening apparatus 72, absorbed into the absorbing liquidand the residue of distillation is discharged as it is.

The distilled water absorbed into the absorbing liquid is caused to flowinto the nitrifying device 20 through the distilled water piping 64 andnitrified through contacting with the fixed media 28, in which AHbacteria grow preferentially, or the fixed media 28, in which AHbacteria and AL bacteria grow mixedly, in the aerobic condition.Furthermore, a storage tank 82 for alkali liquid (for example, weak acidsuch as sodium bicarbonate) for regulating PH in the nitrifying tank 26to about 7˜8 is provided upwardly of the nitrifying device 20, wherebyalkali liquid of a predetermined value is added through an additionregulating valve 68 provided in an alkali liquid piping 66.

The nitrified liquid nitrified in the nitrifying device 20 is caused toflow into the second device 50 for thickening through the nitrifiedliquid piping 34, and alkali is added during this flow by an additionline 84, whereby PH is raised to about 10. The second device 50 forthickening is constituted by a distillation device 86, and theconstruction of the distillation device 86 is similar to that of thedistillation device described in the first form. The nitrified liquid isdistilled in the second device 50 for thickening and is separatedthrough distillation into the distilled liquid, in which the ammoniumnitrogen remaining in the nitrified liquid is thickened, and the residueof distillation into containing the ammonium nitrogen. The distilledliquid is caused to flow into a second absorbing tank 90, in which theabsorbing liquid is stored through an ammonia separating piping 88 andreturned to the nitrifying device 20 intermittently through an on-offvalve 94 provided in an absorbing tank piping 92. On the other hand, theresidue of distillation is discharged as the processed water.

In the case of the system for biologically removing nitrogen of thesecond from as constructed above, the same effect as in the first formcan be obtained. Further, instead of the first and second devices 14 and50 for thickening, the distillation devices 70 and 86 are used, so thatthe lowered processing capacity due to the jamming as in the filmseparation does not occur.

Furthermore, for PH regulation in the nitrifying tank, carbonate such assodium hydrogencarbonate is used in general as described above. However,when the second device 50 for thickening is made to be the filmseparation type, it is essential to provide a solid-liquid separatingtank. However, in the distillation type, it is not necessary to providethe solid-liquid separating tank, so that the device may be made compactin size.

Incidentally, in the above-described forms of operations, when ameasurer for monitoring the ammonium nitrogen of the waste water isprovided to determine the rate of thickening in the firs device forthickening on the basis of the result of measuring, more efficientnitrifying process can be carried out. Furthermore, there is no need formaking the first and second devices for thickening to be identical witheach other, and the film separation type and the distillation type maybe combined freely.

Description will hereunder be given of an embodiment, in which the wastewater of the ammonium nitrogen having the low concentration is nitrifiedand denitrified by use of the system for biologically removing nitrogenequipped with the above-described device for thickening.

(Embodiment 8)

The embodiment 8 shows the case of using the system for processing wastewater as described in FIG. 16, in which the capacity of the nitrifyingtank 26 is set at 5 l. Furthermore, as the fixed media 28, there is useda combining type PVA media, in which the nitrifying bacteria areattached to the surface of the media each having a 3 mmφ for culturing,with the packing rate being set at 20%.

Furthermore, operating conditions were selected by three types Nos. 1 to3, and the residence time in the nitrifying tank 26 was changed so as tomake the load of the ammonium nitrogen per medium reach 600mg-N/·l-medium or more. Furthermore, the concentration of the ammoniumnitrogen of the processed water, the rate of removal of ammonia (therate of removal to the concentration of the ammonium nitrogen flowinginto the nitrifying tank 26) and the nitrifying speed were measured atthe time of stabilization after the continuous operation for 60 days.

According to the No. 1 operating conditions, the waste water of theammonium nitrogen having the ammonium nitrogen concentration of 80 mg/lwas thickened by three times in the first device 14 for thickening,supplied to the nitrifying device 20, the second device 50 forthickening was operated to thicken the waste water by two times, and theresidence time in the nitrifying device 26 was set to one hour.

According to the No. 2 operating conditions, the waste water of theammonium nitrogen having the ammonium nitrogen concentration of 400 mg/lwas thickened by 1.5 times in the first device 14, supplied to thenitrifying device 20, and the second device 50 for thickening wasoperated to thicken the waste water by three times, and the residencetime in the nitrifying tank was set to 5 hours.

According to the No. 3 operating conditions, this is the case where thewaste water of the ammonium nitrogen having the ammonium nitrogenconcentration of a satisfactorily high value of 1000 mg/l. Without usingthe first device 14 for thickening, the waste water was supplied to thenitrifying device 20 through the bypass piping 22 as it was, only thesecond device 50 for thickening was operated to thicken the waste waterby two times, and the residence time in the nitrifying device 26 was setto 6 hours.

As a comparative example, such a case was taken up that the waste waterof the ammonium nitrogen having the ammonium nitrogen concentration of80 mg/l was supplied to the nitrifying device 20 without beingthickened, and only the second device for thickening was operated forthickening the waste water by two times.

FIG. 18 shows the results of above-described operating conditions andthe results of the experiments.

As the result, the following are the results of measuring the ammoniumnitrogen concentration of the respective effluent waters (the processedwaters through the second device for thickening) in Nos. 1 to 3. In No.1, 4 mg/l (rate of removal . . . 98.3%) was attained, in No. 2, 8˜9 mg/l(rate of removal . . . 98.7%) was attained, and in No. 3, 10 mg/l (rateof removal . . . 99.0%) was attained, so that satisfactory decreasingwas attained. In contrast thereto, the ammonium nitrogen concentrationof the processed water in the comparative example was 10 mg/l (rate ofremoval . . . 87.5%), so that the rate of removal was low as comparedwith Nos. 1 to 3.

Furthermore, all of the results of measuring the nitrifying speeds inNos. 1 to 3 were 400 mg-N/h·l-medium or more, and No. 3 showed a veryhigh value of 460 mg-N/h·l-medium. In contrast thereto, the nitrifyingspeed in the comparative example was 120 mg-N/h·l-medium, which wasabout 1/4 in the cases of Nos. 1 to 3.

Furthermore, when the nitrifying bacteria contained in the media 28after the completion of the examination were fixed, AH bacteria and ALbacteria were ascertained to be alive in Nos. 1 and 2 media 28, and itwas also ascertained that, in the media 28 of No. 3, AH bacteria grewpreferentially. Furthermore, in the media 28 of the comparative example,most of the bacteria were AH bacteria, and it is deemed that thedifference in type of bacteria had been reflected in the nitrifyingspeed.

(Embodiment 9)

The embodiment 9 shows the case of using the system for processing thewaste water as described in FIG. 17. The capacity of the nitrifying tank26, the fixed media 28 which were used, and the packing rate of thefixed media 28 are the same as those shown in the embodiment 1.Furthermore, in this embodiment, there was used the waste water of urinesynthesized as the waste water of the ammonium nitrogen and diluted tothe extent where the concentration of the ammonium nitrogen was 80 mg/l.Furthermore, the concentration of the ammonium nitrogen of the processedwater and the nitrifying speed were measured at the time ofstabilization after the continuous operations for 60 days.

Operating condition is in this embodiment were selected such that thewaste water of the ammonium nitrogen having the concentration of theammonium nitrogen of 80 mg/l was thickened by five times in the firstdevice 14 for thickening, supplied to the nitrifying device 20, thesecond device 50 for thickening was operated to thicken the waste waterby two times, and the residence time in the nitrifying device 26 was setto 4 hours.

As a comparative example, such a case was taken up that the waste waterof the ammonium nitrogen having the ammonium nitrogen concentration of80 mg/l was supplied to the nitrifying device 20 without beingthickened, only the second device 50 for thickening was used to thickenthe waste water by two times, and the residence time in the nitrifyingdevice was set to 20 hours.

FIG. 19 shows the above-described operating conditions and the resultsof the experiments.

As the result, in the case of this embodiment, the ammonium nitrogenconcentration of the residue of distillation discharged from the firstdevice 14 for thickening was less than 5 mg/l. Furthermore, the ammoniumnitrogen concentration of the processed water (residue of distillation)discharged from the second device 50 for thickening was stabilized at avalue less than 10 mg/l. Furthermore, the nitrifying speed reached 290mg-N/h·l-medium. Thus, the high nitrifying speed was attained in thisembodiment and the processing at high speed was performed similarly tothe embodiment 8.

On the other hand, in the case of the comparative example, the residencetime in the nitrifying tank 26 was set at a value five times larger thanthat in this embodiment, whereby the ammonium nitrogen concentration ofthe processed water (residue of distillation) reached a value less than10 mg/l. However, the nitrifying speed was decreased to a very low valueof 14 mg-N/h·l-medium as compared with this embodiment, which is far offin result from the processing at high speed. It is deemed that thedecrease in the nitrifying speed was caused by the decrease in thedensity of bacteria in accordance with the ammoniac concentration in thenitrifying tank 26.

As described above, in the method of biologically removing nitrogen andthe system therefor according to the present invention, by utilizing thecharacteristics of AH bacteria as being the nitrifying bacteria ofnitrous acid type for displaying high activity at the high concentrationof the ammonium nitrogen and the characteristics of AL bacteria as beingthe nitrifying bacteria of nitrous acid type for displaying highactivity at the low concentration of the ammonium nitrogen, in thenitrifying process, the operations capable of obtaining the maximumnitrifying speed in accordance with the ammonium nitrogen concentrationcan be performed and the nitrifying reaction of nitrous acid type forproducing nitrous acid as being the intermediate oxide material of theammonium nitrogen can be performed.

On the other hand, in the denitrifying process, the nitrified liquid, inwhich the rate of nitrous acid is high, is denitrified, whereby thenitrous acid reducing bacteria as being the denitrifying bacteria of thetype of reducing nitrous acid grow preferentially, so that thedenitrifying reaction for reducing nitrous acid into nitrogen gas can beperformed efficiently.

With this arrangement, the waste water of the ammonium nitrogen havingthe high concentration can be processed at high speed without dilutingit and nitrous acid as being the intermediate oxide material can bedenitrified at the stage of nitrous acid, so that the period of timerequired for denitrifying process can be shortened. Accordingly, thesystem can be made compact in size even in the case of the waste waterof the ammonium nitrogen having the high concentration, so that thissystem is particularly effective as the system for biologicallyprocessing in he places where the waste water of ammonium nitrogenhaving the high concentration on the level of 400 mg/l˜5000 mg/l isproduced in great quantity such for example as the developinglaboratory, the inorganic matter synthesizing factory and the powerplant.

Further, the waste water of the ammonium nitrogen having the lowconcentration can be thickened and processed, so that the quantity ofprocessing of the waste water in the case, where the waste water of theammonium nitrogen having the low concentration as in the sewerage isprocessed, can be decreased greatly. Accordingly, the system can be madecompact in size.

It should be understood, however, that there is no intention to limitthe invention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications, alternate constructions andequivalents falling within the spirit and scope of the invention asexpressed in the appended claims.

We claim:
 1. A system for biologically removing nitrogen, wherein liquidcontaining ammonia is brought into contact with nitrifying bacteria in anitrifying portion in the aerobic condition and brought into contactwith denitrifying bacteria in a denitrifying portion in the anaerobiccondition, to thereby remove ammonium nitrogen in the liquid containingammonia,characterized in that said nitrifying portion includes: aflow-in portion of said liquid containing ammonia; an AH bacteriareacting portion containing AH bacteria fixed media of floating type, inwhich AH bacteria as being nitrifying bacteria grow preferentially, forbringing said liquid containing ammonia, which has flowed in from saidflow-in portion, with said AH bacteria fixed media in the aerobiccondition; and an AH-AL bacteria reacting portion containing AH-ALbacteria fixed media of floating type, in which AH bacteria and ALbacteria as being nitrifying bacteria grow mixedly, for bringing liquidwhich has flowed out of said AH bacteria fixed media into contact withsaid AH-AL bacteria fixed media in the aerobic condition.
 2. A systemfor biologically removing nitrogen, wherein liquid containing ammonia isbrought into contact with nitrifying bacteria in a nitrifying portion inthe aerobic condition and brought into contact with denitrifyingbacteria in a denitrifying portion in the anaerobic condition, tothereby remove ammonium nitrogen in said liquid containingammonia,characterized in that said nitrifying portion includes: aflow-in portion of said liquid containing ammonia; an AH bacteriareacting portion containing AH bacteria fixed media of floating type, inwhich AH bacteria as being nitrifying bacteria grow preferentially, forbringing said liquid containing ammonia, which has flowed in from saidflow-in portion, into contact with said AH bacteria fixed media in theaerobic condition; an AH-AL bacteria reacting portion containing AH-ALbacteria fixed media of floating type, in which AH bacteria and ALbacteria as being nitrifying bacteria grow mixedly, for bringing liquid,which has flowed out of said AH bacteria reacting portion, into contactwith said AH-AL bacteria fixed media in the aerobic condition; and an ALbacteria reacting portion containing AL bacteria fixed media of floatingtype, in which AL bacteria as being nitrifying bacteria growpreferentially, for bringing liquid, which has flowed out of said AH-ALbacteria reacting portion, with said AL bacteria fixed media.
 3. Asystem for biologically removing nitrogen, wherein liquid containingammonia is brought into contact with nitrifying bacteria in a nitrifyingportion in the aerobic condition and brought into contact withdenitrifying bacteria in a denitrifying portion in the anaerobiccondition, to thereby remove ammonium nitrogen in said liquid containingammonia,characterized in that said nitrifying portion includes: aflow-in portion of said liquid containing ammonia; an AH-AL bacteriareacting portion containing AH-AL bacteria fixed media of floating type,in which AH bacteria and AL bacteria as being nitrifying bacteria growmixedly, for bringing liquid, which has flowed in from said flow-inportion into contact with said AH-AL bacteria fixed media in the aerobiccondition; and an AL bacteria reacting portion containing AL bacteriafixed media of floating type, in which AL bacteria being nitrifyingbacteria grow preferentially, for bringing liquid, which has flowed outof said AH-AL bacteria reacting portion, into contact with said ALbacteria fixed media.